After leaving Trinity College, Cambridge, Elliott was offered a post in 1938 as Honorary Attache at the Hague by Sir Nevile Bland. His career in secret intelligence came by chance, like many before and after him. Sir Hugh Sinclair, Head of MI6, happened to visit the Hague, took to Elliott and offered him a job.

In 1943, he married Elizabeth Holberton, with one son and one daughter, deceased.

Elliott's intelligence career was marked by two publicly noted events, the death of Commander Lionel Crabb and the flight of traitor Kim Philby to Moscow. Elliott and MI6 suffered criticism in both cases and he felt this deeply to the end of his life.[1]

In 1956, during Khrushchev's visit to Britain, the Soviet Sverdlov class cruiser Ordzhonikidze visited Portsmouth Harbour. The Royal Navy was interested in anti-submarine warfare equipment carried under the cruiser's stern. Elliott arranged for Crabb, an experienced ex-naval frogman, to investigate. He made one successful run under the ship, came back for an extra pound of weight for his next attempt and failed to return from the second dive. Elliott speculated in his autobiography that Crabb suffered equipment failure. Subsequent criticism of Crabb, whom Elliott believed to be a brave and honourable officer and who had undertaken successful operations of the same kind before, was resented by Elliott.

The Soviets, who had reported a diver in trouble near the stern, did not complain, but also denied responsibility for Crabb's death. But the matter leaked. Prime MinisterAnthony Eden protested that he had not been informed and adverse publicity ensued. Elliott claimed he had been told the operation had been cleared by the Foreign Office.

In With My Little Eye, Elliott gives an account of his last contacts with Kim Philby, in 1963.[1] Philby, with whom Elliott had worked in Beirut, had been a friend, and Elliott felt his betrayal bitterly. He volunteered to confront Philby, in an effort to obtain a full written confession of his espionage. Though Philby did confess in person to Elliott, he delayed signing a written confession and, instead, immediately fled to Moscow, where he was granted Soviet citizenship. Public criticism of MI6, which had failed to guard against his escape, was significant. However, Elliott felt he could not have prevented Philby's flight.

According to author Ben Macintyre, who wrote a book on Philby, it is possible that Philby was allowed to flee to Moscow to avoid an embarrassing trial; and he records suspicions that a typical British Establishment old boy network had intervened for the mutual convenience of MI6 and Philby.[3]
Philby himself also speculated that this was the case, according to Macintyre.